1. Field of the Invention
This invention relates to a device for excitation of a gas column enclosed in a hollow-core optical fiber that makes it possible to obtain in particular a light source or a laser source.
2. Description of the Related Art
In the field of laser sources, there are several families:
The first family comprises laser diodes that are compact and inexpensive. However, these laser sources are limited to fragmented spectral ranges of conventional wavelengths ranging from 0.4 μm to 2 μm. This type of laser can be used, for example, as a pump source for bulkier lasers such as crystalline-type lasers.
The second family comprises crystalline-type or ion-type lasers that use a solid medium such as an emission medium, doped by ions. This type of laser is relatively expensive and bulky, and the emission wavelengths are limited to radiative transitions of ions (in particular rare earth ions) doping the solid medium and/or the emission spectrum of the solid medium.
The third family comprises gas lasers whose purpose is to excite a gas column enclosed in a tube or in a tank. According to the prior art, this type of laser has the advantage of creating a beam emitting at wavelengths that are inaccessible by the solid lasers cited above. In particular, the wavelengths of gas lasers can be in the ultraviolet range for the excimer lasers, in the visible range for the argon lasers, HE-NE, and in the infrared range for the CO2 lasers. For this type of laser, the wavelength of the beam that is created depends on the composition and the pressure of the gas mixture present in the tube or in the tank.
Even if this laser design makes it possible to obtain beams with wavelengths within a broad spectrum, it is not fully satisfactory essentially for two reasons.
In a gas laser, the electroluminescent discharge originates from a longitudinal electrical field between two electrodes arranged in the interior and at each of the ends of a tube containing gas. The electrodes that are placed in the gas tend to corrode and to contaminate the medium. Consequently, it is necessary to clean these electrodes frequently, which tends to greatly increase the operating costs of such a laser.
According to a second drawback, the radiation emitted by this type of laser is not guided but is in free space. Consequently, it is necessary to provide a complex set of optical elements for moving the light beam toward its target, which leads to complex and bulky systems, difficult to regulate and to maintain. Finally, the configuration of interaction in free space between the excitation and the gas greatly limits the effectiveness of the laser (low optical yield).
Taking into account the primary characteristics of the lasers of the prior art, certain industrial needs are not met to the extent that there is no compact and inexpensive laser in the manner of a diode laser, able to emit a beam with an unconventional wavelength, in particular in the ultraviolet range, in the manner of a gas laser.
Taking into account the capacity of the gas lasers to create light beams with wavelengths within a broad spectrum, the purpose of the invention is to propose a gas laser source that is not very bulky and that has both a reduction in operating costs and a drastic increase relative to the optical yield.
In the field of plasmas, a device for excitation of a gas column enclosed in a tube, called “Surfatron,” without electrodes placed in the interior of the tube is known. Such a device is described in particular in the patent FR-2,290,126.
According to this document, the gas mixture is placed in a hollow tube with a minimum diameter of 2 mm. The excitation device comprises a metal chamber, coaxial to the tube, bounded by a first cylindrical wall whose inside diameter is adjusted to that of the tube, a second cylindrical wall that is spaced and coaxial to the first cylindrical wall, and two side walls. According to a characteristic of this device, an annular space is made between the first cylindrical wall and a side wall that is said to be thin since it is less thick than the other side wall. In addition to the metal chamber, the excitation device comprises a coupling element in the form of a metal plate arranged in the chamber close to the first cylindrical wall and the annular space. The coupling element is connected to a feed that can provide a high-frequency excitation signal via a coaxial cable of which one thread is connected to the metal plate and the other to the second cylindrical wall.
According to this document, by providing a signal with a frequency that is between 100 and 1,500 MHz, it is possible to create in the annular chamber an electrical field whose direction close to the annular space is parallel to the shaft of the tube that contains the gas and creates surface waves that are able—if the power of the feed is adequate—to ionize the gas contained in the tube.
According to this document, this type of excitation device combined with a tube that is closed at each end containing a gas can constitute a light source. In turn, this light source can be used as an exciter of a solid medium for forming an ion-type laser.
The device described in the document FR-2,290,126 has the drawback of emitting an unguided radiation (in free space), with the function of the tube being limited to the role of chamber for gas and plasma.
Another excitation device described in the patent FR-2,579,855 was developed so as to obtain a laser effect by exciting a gaseous medium.
According to this document, the gas mixture is stored in a tube with an inside diameter on the order of 1.5 mm, and the excitation device comprises a microwave applicator that is able to create, on the exterior of the tube containing the gas, surface waves that can produce plasma in the interior of the tube.
The device that is described in this document corresponds to an objective of the invention, namely to provide an excitation device that can create a laser effect in a gaseous medium that can therefore—based on the composition and the pressure of the gas—have an unconventional wavelength.
However, the device illustrated in the patent FR-2,579,855 is not compact since the excitation device extends over a length on the order of 14 cm.
According to another problem, the applied microwave power is relatively high and greater than 200 watts. Finally, the radiation that is emitted by this device is not guided but is in free space, with the function of the tube proposed in the patent FR-2,579,855 being limited to the role of chamber for gas and plasma.
The document US-2007/0280304 describes a device for excitation of a gaseous medium contained in a casing whose diameter is between 500 and 750 μm. The excitation device comprises a system that is arranged on the exterior of the casing that can create an electromagnetic field at the level of the gaseous medium. This excitation system comes in the form of a coil wound around the casing. This excitation mode does not offer any particular control over the plasma that is created. Consequently, this embodiment is not of high quality because it does not comprise any means for optimizing the coupling of the electromagnetic excitation with the gaseous medium, although the risks of deterioration of the casing because of the high temperatures of the plasma are very high.